AISHA IIAircraft Integrated Structural Health Assessment II

State of the Art - Background

Today's aircraft inspection procedures are excellent, but too
expensive and not appropriate for all situations. A cheaper
alternative for damage detection is offered by 'structural health
monitoring'(SHM). With such SHM systems, a permanent sensor
network, comparable to the nervous system in a human body, is
placed at crucial structural components of an aircraft. In this
project, guided ultrasonic waves (Lamb waves) are used to detect
different kinds of defects.

However, this apparently easy solution requires a quite complex
research and implementation effort using well-coordinated
collaborations of many disciplines and expertise in Europe. The
European Research Area establishes the ideal platform for such a
collaborative undertaking, and the considerable financial risks can
be reduced by a substantial amount by appropriate funding from
dedicated European research programmes.

During the last few years, a number of SHM solutions have been
presented at laboratory scale, and even partially implemented in
real aircraft parts. There is thus, in principle, enough
experimental evidence that such systems are able to deliver all the
required information. However, the final implementation is still in
an early phase and is partially hindered by a number of obstacles
(technical immaturity, lack of acceptance by end-users, etc.).

Objectives

It will not be possible within this project to present a
large-scale integrated technical solution to overcome all the
challenges related to structural health monitoring. However, the
following strategies have been chosen to put the operating systems
into practice.

It appears that the best strategy to overcome the
above-mentioned obstacles is to develop a SHM system for selected,
isolated problems which can easily be followed and validated by
conventional methods. If this technique appears to be trustworthy,
a broader field of application will be created.

AISHA II therefore intends to focus on hot-spot monitoring
instead of large-area screening. From operational experiences it is
known that defects usually occur at well-defined locations. There
are thus isolated problems in maintenance where a simple SHM can
give reasonable added value, without screening the whole
aircraft.

Description of Work

This project will use a limited number of carefully selected
ultrasonic Lamb wave modes in the detection process. Lamb mode
selection, both active and passive, is, however, not an easy
process. As one of the main innovative aspects, the principle of
controlled Lamb wave selection will be used as the basis of an
aircraft health monitoring system, both for active and passive Lamb
wave testing. In practice, this will mean that the amount and type
of Lamb wave modes to be used in the monitoring process will depend
on the type of material and damage to be detected and will thus
have to be controllable by the user. Other NDT technologies will
also be applied, such as electrochemical monitoring and eddy
current.

Other work includes:

- opening the initial phase with the establishment of detailed
specification sheets where the different demands on damage
detection are clearly defined. The respective full-scale part will
be studied by the assigned NDT groups, the different aspects of
feasibility explored and the final plan for SHM implementation
developed;

- implementing the selected SHM systems will be carried out in
close collaboration with all partners. Using the transducer,
hardware and software required, the approved concepts will be
implemented following the detailed road map defined in the design
phase;

- an extended test programme will be run to check all
operational aspects.

Expected Results

The expected progress that the proposed project will bring with
respect to the state of the art will be the following:

- A selection of very specific aircraft components (representing
isolated 'hot spots') in collaboration with the aircraft operators
and aircraft manufacturers. The expected cost savings from using
SHM must be considerable;

- A systematic research on durable sensor connections ensured by
collaborating with a specified research institute;

- If proved useful, the introduction of the pseudo-defect
technique for automated validation. This enables a dramatic
enhancement in the efficiency of the validation tests leading to a
fine-tuning of data analysis techniques;

- The application of combined sensor groups (ultrasonic sensor +
parametric sensors for temperature and strain), electrochemical
monitoring and thermography. These techniques are beyond the
application of ultrasonic Lamb waves, but they help to facilitate
the interpretation of signal-damage relationships.